This invention pertains to self-excitation of the alternator in a motor vehicle charging system, and more particularly, to a self-excited charging system using an alternator of the low-carbon steel rotor type and an electronic voltage regulator which is sufficiently sensitive to respond to electrical signals developed from residual magnetism in the alternator to self-excite the alternator at low engine RPM.
One technique known to the prior art to excite an alternator is to provide an excitation current or the like into the field winding of the alternator during start-up or during low engine RPM. Such an arrangement, however, frequently requires an extra set of contacts in the ignition switch which necessarily adds to the cost of the charging system and also adversely affects charging system reliability. An extra wire or connection in the charging system is also frequently required when using this technique.
Other techniques have also been used to self excite the alternator of the charging system and to minimize the number of connections in the charging system. Another such technique is the use of high carbon steel in the rotor core of the alternator. The high carbon steel provides more residual magnetism and therefore a correspondingly higher electrical signal during start-up of the alternator which may then be utilized to self-excite the alternator. However, high carbon steel in the rotor lowers the permeability of the rotor iron. This lower permeability means that there will be less magnetic flux for given conditions compared to the use of low carbon steel. The lower permeability of high carbon steel in the rotor also necessarily means that the output current capability of the alternator is reduced compared to the use of a similar amount of low carbon steel. The output of the high carbon steel type rotors may of course be increased by adding extra steel, but this is under a weight and cost penalty in order to provide the same output current capability.
High carbon steel rotor cores are used in the prior art to take advantage of the residual magnetism present in the alternator to provide a signal of sufficient magnitude to overcome the input threshold in the voltage regulator for the alternator and/or the forward bias potential of the typical diode trio. The voltage regulator is typically referenced to ground and is biassed from the battery which is most commonly a positive voltage supply. Voltage regulators therefore typically have an input threshold voltage which must be reached and exceeded in order to provide an output signal suitable for energizing the alternator field coil. Typically, the voltage signal resulting from the residual magnetism is rectified as by a diode trio, and then supplied to the input of the voltage regulator. This means that there is at least one forward biassed silicon diode drop in the diode trio, which is typically 0.7 volts for a silicon diode. Additionally, as mentioned above, the voltage regulator input may also have some threshold level before activation. Typically, the input stage to a voltage regulator has an NPN type transistor with its emitter referenced to ground and the base terminal thereof coupled to the input terminal of the voltage regulator. This means that yet another semi-conductor must be forward biased in order to activate the electronic voltage regulator. For an NPN type transistor with its emitter referenced to ground and the base terminal coupled to the input terminal of the voltage regulator, another 0.7 volt drop will be needed to forward bias the base to emitter junction of the NPN transistor. These voltage drops necessarily translate into significant engine RPM before the residual magnetism in the alternator will provide a sufficiently large signal that may be rectified and applied to the input of the voltage regulator to activate the regulator. For example, in self-exciting charging systems utilizing an alternator of the high carbon steel rotor type, it is not at all unusual for engine RPM to have to exceed 1,000 before the voltage regulator will sufficiently energize the field winding of the alternator and enable the alternator to cut in. Since approximately a 2:1 pulley ratio is commonly employed between the engine and the alternator, 1000 engine RPM translates into about 2000 alternator RPM. If the engine of the charging system in which the alternator is installed should hesitate or otherwise begin to stall, the engine RPM may drop sufficiently that the alternator may cut out and the minimum engine speed for alternator cut in may again have to be exceeded before the charging system is again self-excited.